Field of the Invention
The present invention pertains to a CO.sub.2 foam drive process for recovering oil from a subterranean oil-containing formation. More particularly, the present invention involves an improved CO.sub.2 foam drive and recovery method from a subterranean formation penetrated by at least one injection well and at least one spaced-apart production well wherein the formation around the injection well is fractured before the CO.sub.2 foam is injected into the formation for oil recovery.
A variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean formations. These techniques include thermal recovery methods, waterflooding and miscible flooding.
Fluid drive displacement of oil from an oil-containing formation utilizing CO.sub.2 is known to have the following effect in enhancing the recovery of viscous oils: (1) oil swelling, (2) viscosity reduction; and (3) when dissolved in an aqueous driving fluid it dissolves part of the formation rock to increase permeability. As the oil viscosity increases, a straightforward CO.sub.2 immiscible flood becomes less effective because of gravity override and viscous fingering due to unfavorable mobility ratio as disclosed in the article by T. M. Dosher et al, "High Pressure Model Study of Oil Recovery by Carbon Dioxide", SPE Paper 9787, California Regional Meeting, Mar. 25-27, 1981. It is known that the oil displacing efficiency of a CO.sub.2 drive can be improved by mixing the CO.sub.2 with a foaming agent to produce a CO.sub.2 foam oil recovery driving fluid. The foam is effective at controlling CO.sub.2 channeling due to stratification and fingering. In addition, the foam also effectively reduces the mobility of CO.sub.2 in porous media and controls CO.sub.2 injection profiles, resulting in increased oil recovery and sweep improvements. Numerous patents have been issued on the recovery of oil using a CO.sub.2 foam drive which include U.S. Pat. Nos. 3,330,346; 4,113,011; and 4,380,266. In addition, U.S. Pat. No. 4,577,688 discloses the use of steam, CO.sub.2 and a foaming agent in an oil recovery process.
The present invention more effectively utilizes the CO.sub.2 foam in a CO.sub.2 foam enhanced oil recovery process by first fracturing the oil-containing formation around the injection well so that the subsequent injection of CO.sub.2 foam can more effectively penetrate the formation resulting in enhanced oil recovery and a reduction in the amount of CO.sub.2 required.